Actress Barbara Windsor will take to the tannoy at Tube stations on London Poppy Day to raise money for the Royal British Legion. She will ask the public to donate money for a poppy.
Windsor said: "We want this year desperately to get to a million."
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.